Sorry for the downtime. I have upgraded the server packages and Google offered a new pagespeed module.
Of course I said yes, since we are on the "stable" channel of the mod_pagespeed repo.
So after the package got installed, it turned out, Google removed an option from the config files.
Instead of displaying a warning like "it's deprecated", it just stopped the webserver.
Feel free to mail me if something like this happens. (The site should never go offline without a notice.)